No, no, you're right abotu position being a factor. When you use teh Spec pad, you lean forward or not, to approximate the position you will be in on the bike. This pelvic rotation presents the representative portion of your ischia tuberosities to the pads. The recommendation one may get from this test shouldn't be taken out of this context. And while I believe nearly nothing happens in a bike shop that is not a sales ploy this is a little less so, since the numerical sizings hold true for Specialized saddles of all price ranges. If you're 143 on your sport bike, you're a 143 whether you buy an Alias or a Toupe. If the salesman tries to sell you a Toupe, it's, well, who can blame him. Like with women, can you blame a guy for trying? Caveat emptor!

@ the OP: these width measurements have nothing to do with Brooks saddles. The system is only designed for Specialized saddles, by Specialized. Specialized saddles do run true to size, but they are just not Brooks sizes. Probably the best way to deal with width sizing of Brooks saddles is to choose a starting point and evaluate and experiment from there. You can choose a starting point based on the aggressiveness of your preferred position. Handlebars more than a few cm lower than the saddle, I'd suggest a Pro, B17N, or Swift. Handlebars nearly even: a B17, Imperial or not. Handlebars more than a few cm above saddle, maybe a B72 or whatever the modern incarnation is. The saddle is made wider the more upright the positioning is.

With any trial of a Brooks saddle, your results may greatly depend on subtle positioning; up/down, fore/aft, tilt nose up or nose down. Don't just bolt it on and expect Nirvana. This applies to Spec's, too.

Brooks true to size? Yes, true to their sizes.